Ok need to get better advise on this after the holiday but don't think a 2 pole can be changed into an 8 pole just by winding.
OK the stator has 24 slots but the rotor has to match.

Is there an easy way to find out the slots/bars in the rotor, the laminations are ground smooth and appear continuous, it is 58 dia x 130 long, the stator is 108 o/d

Also at 8 pole you will get approx 5.700 revs at 400 hz and there will be a limit you can go to on hz before it all gets saturated or egg bound in technical terms.

Can't find the specs for my exact motor but HSD do give figures for their 4 pole motors to 28000 rpm (933hz). It is 5 Kw continuously rated so I was hoping that derating to ~3 Kw would give a bit of headroom. Because its a mill I am more interested in some low speed torque than high speed, I think ~6000 rpm max would be OK